1. Field of the Invention
The present invention relates to an active matrix type display apparatus wherein a display screen is configured by arranging display pixels including self-luminescent elements such as, for example, an electroluminescence (hereinafter, referred as EL) element in a matrix form.
2. Description of the Related Art
Flat panel type display apparatuses have been broadly used as a display apparatus for a personal computer, an personal digital assistant, a television, or the like. In recent years, as such a flat panel type display apparatus, an active matrix type organic EL display apparatus using a self-luminescent element such as an organic EL element has been given the attention, and the research and development thereof have been actively carried out. The organic EL display apparatus has the following features: it does not require a backlight preventing the organic EL display apparatus from being made to be thin and light-weight, it has a high-speed responsiveness and is suitable for playing-back moving picture, and moreover, it can be used at cold districts as well, because the brightness thereof is not reduced at a low temperature.
Generally, the organic EL display apparatus comprises a plurality of display pixels which are arranged in a plural rows and a plural columns to constitute a display screen, a plurality of scanning lines extending along the respective rows of the display pixels, a plurality of signal conductor lines extending along the respective columns of the display pixels, a scanning line driving circuit for driving the respective scanning lines, a signal conductor line driving circuit for driving the respective signal conductor lines, and the like. Each display pixel includes an organic EL element which is a self-luminescent element, and a pixel circuit for supplying a driving electric current to the organic EL element. The pixel circuit has a pixel switch disposed in the vicinity of the cross positions of the scanning lines and the signal conductor lines, a driving transistor which is connected in series to the organic EL element between a pair of power source lines, and which is formed of a thin-film transistor, and a storage capacitance retaining the gate control voltage of the driving transistor. The pixel switch is made to be conductive in response to a scanning signal supplied from a corresponding scanning line, and acquires an image signal supplied from a corresponding signal conductor line into the pixel circuit. The image signal is written as the gate control voltage into the storage capacitance, and is stored for a predetermined period. The driving transistor supplies an electric current amount corresponding to the gate control voltage written in the storage capacitance to the organic EL element, and the organic EL element is operated to emit light.
The organic EL element has a cathode, an anode, and an emitting layer which is formed of a thin-film including a fluorescent organic compound and provided between the cathode and the anode. The organic EL element generates an exciton by injecting electrons and holes into the emitting layer and recombining those, and emits light due to the light emission generated at the time of deactivation of the exciton. The organic EL element emits light at a brightness corresponding to a supplied electric current amount, and a brightness of about 100 to 100,000 cd/m2 can be obtained by even an applied voltage equal to or less than 10 V.
In the organic EL display apparatus, a thin-film transistor serving as the driving transistor has a semiconductor thin-film formed on an insulating substrate such as a glass. Therefore, the characteristics of the driving transistor such as a threshold voltage Vth and a carrier mobility μ depend on the manufacturing process or the like, and easily vary. If there is unevenness in the threshold voltage Vth of the driving transistor, it is difficult to make the organic EL element emit light at an appropriate brightness. Thus, an irregularity in brightness among the plurality of display pixels arises, which causes unevenness in displaying.
For example, in U.S. Pat. No. 6,229,506, there is disclosed a display apparatus in which threshold canceling circuits are provided at all of display pixels in order to avoid the effect due to the irregularity in the threshold voltage Vth. Each threshold canceling circuit is configured such that the control voltage of the driving transistor is initialized by a reset signal supplied in advance of an image signal from the signal conductor line driving circuit. Further, as the other display apparatus, in U.S. Pat. No. 6,373,454, there is proposed a display apparatus in which writing of an image signal is carried out by an electric current signal, and an attempt is made to uniform the brightness of light emission by reducing the effect due to the irregularity in the threshold voltage in the driving transistor.
In the display apparatus described above, the pixel circuit of each display pixel has one or more switches which are connected between a gate and a drain of the driving transistor, and which are in OFF-states for a period of light-emitting, and the switches are respectively formed of thin-film transistors. However, in such a pixel circuit, when these switches are switched from being on to being off, feedthrough voltages due to the parasitic capacitance formed between the gates and the sources of the switches are generated. Further, a gate control voltage of the driving transistor is varied by an amount corresponding to the amount of the generated feedthrough voltages. Because the feedthrough voltage depends on a threshold voltage of the switch, an irregularity arises in the gate control voltage of the driving transistor due to the irregularity in the threshold voltage, and an irregularity in brightness arises among the plurality of display pixels. Such an irregularity in brightness among the display pixels appears as the unevenness in displaying, which deteriorates the quality of displaying.
For example, when the above-described switches and the driving transistor are formed of P-channel type thin-film transistors, the gate control voltage of the driving transistor is varied in plus electric potential direction, and electric current made to flow in the driving transistor changes for being reduced. This leads to a reduction in EL light emitting current, which decreases white brightness on a displayed image.
By supplying from the driving circuit an image signal to which an amount of the reduction in light emitting current is added in advance, the problem of the insufficient white brightness can be avoided. However, in this case, a rise in a driving voltage of the driving circuit, upsizing of the driving circuit, an increase in manufacturing cost, or the like are brought about.